1. Field of Invention
The invention relates to a zoom lens system for a compact video camera, an electronic still camera, etc. and more particularly to such a zoom lens system having a first lens group of positive diopter, a second lens group of negative diopter, and a third lens group of positive diopter.
2. Description of Related Art
Recently, compact zoom lens systems as the most important component for a compact video camera, an electronic still camera, etc. are developed and commercially available. A compact zoom lens has the benefits of decreasing weight and greatly reducing cost. As a result, for example, the size, weight, and unit price of a compact video camera are decreased greatly.
A typical zoom lens system has a first lens group of positive diopter, a second lens group of negative diopter, and a third lens group of positive diopter. Position of the first lens group of positive diopter is fixed during zooming and focusing. To the contrary, the second and third lens group of diopters are changed along an optical axis.
There have been numerous suggestions in prior patents (e.g., U.S. Pat. Nos. 5,268,793, 5,627682, and 5,543,969) for zoom lens systems. In U.S. Pat. No. 5,543,969, OAL/IMA of the zoom lens (i.e., ratio of total track of zoom system to image sensor size) is about 22. In U.S. Pat. No. 5,627,682, OAL/IMA is about 21. In U.S. Pat. No. 5,268,793, OAL/IMA is higher than 26. It is known that the diameter of zoom lens is mainly determined by image sensor size and zoom ratio. Thus, the smaller of OAL/IMA (i.e., ratio of total track of zoom system to image sensor size) the better of the image quality will be. However, each of the above patents is not desirable due to its high such ratio. Thus, the need for improvement still exists.